The invention relates to a miniature electric circuit breaker with an insulating moulded casing having an arc formation chamber containing a pair of separable contacts, an arc extinction chamber communicating with the arc formation chamber and a housing chamber for the mechanism, separated from the arc formation chamber by an intermediate insulating partition wall. The wall has a rectangular passage slot of the support arm of the moving contact, the slot being elongated in the moving direction of the arm and laterally delimited by two insulating ribs of the partition encompassing the arm with weak play and extending until the side walls of the moulded casing.
When a short-circuit is cut off, the arc which develops in the arcing chamber releases much heat and damages the casing parts located close to its passage. Furthermore the arc roots pull some metallic particles out of the contacts and the arcing horns. The degradation products, composed mainly by carbon, metals and oxydes form preferentially a deposit on the ribs and in the angles, wherever the gaseous flow which sweeps them along is deviated by an obstacle. In particular they form a deposit on the intermediate partition between the arc formation chamber and the chamber containing the mechanism. In all the known realizations this partition with slot extends between the casing parts near the fixed contact and the casing parts near the moving contact at the shortest. This arrangement has the disadvantage to create a preferential creepage path between the fixed contact and the moving contact which degrades the insulation level of the appliance.